Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene

Abstract

Twisted double bilayer graphene (tDBG) comprises two Bernal-stacked bilayer graphene sheets with a twist between them. Gate voltages applied to top and back gates of a tDBG device tune both the flatness and topology of the electronic bands, enabling an unusual level of experimental control. Metallic states with broken spin and valley symmetries have been observed in tDBG devices with twist angles in the range 1.2–1.3°, but the topologies and order parameters of these states have remained unclear. We report the observation of an anomalous Hall effect in the correlated metal state of tDBG, with hysteresis loops spanning hundreds of mT in out-of-plane magnetic field (B⊥) that demonstrate spontaneously broken time-reversal symmetry. The B⊥ hysteresis persists for in-plane fields up to several Tesla, suggesting valley (orbital) ferromagnetism. At the same time, the resistivity is strongly affected by even mT-scale values of in-plane magnetic field, pointing to spin-valley coupling or to a direct orbital coupling between in-plane field and the valley degree of freedom.

Non-coplanar magnetism, topological density wave order and emergent symmetry at half-integer filling of moiré Chern bands

“…Note added: In the final steps of preparation of this manuscript, experimental signatures of an anomalous Hall effect in ABAB stacked TDBG near ν " 7{2 and its stability under both inand out-of-plane magnetic fields were reported in Ref. [62]. This is consistent with our results for TDBG and strengthens the established similarity of TDBG and TMBG in that it is in line with the observation of symmetry broken Chern insulators at ν " 7{2 in Ref.…”

Observation of room temperature anomalous Hall effect in graphene-WSe2 heterostructures

“…The study in corrugated systems is particularly fascinating as it demonstrates the appearance of a linear Hall response even under time-reversal symmetric conditions for systems with tilted bands in a reduced-symmetry scenario. More recently, AHE has been observed in graphene-based moiré heterostructures at half-or quarter-filling of the bands owing to the spontaneously broken time-reversal symmetry and magnetization arising from the enhancement of the exchange interactions by the large density of states of the flat bands [26][27][28][29][30][31][32][33] .…”

Symmetry-Broken Chern Insulators in Twisted Double Bilayer Graphene

“…12 Compared with twisted bilayer graphene (tBLG), in which strongly correlated states occur over a very narrow range near the magic angle of ≈1.1°, tDBG hosts correlated states over a wider range of twist angles due to its lack of a singular "magic angle" from theory. A wide variety of correlated phenomena have been reported already in tDBG, including states that are spin-polarized, 4,5 valleypolarized, 13 spin−valley-polarized, 15 and intervalley-coherent, 16 as well as a correlated electron−hole state, 8 the anomalous Hall effect (AHE), 14 and superconductivity in samples interfaced with WSe 2 . 17 Despite this, the phase diagram with respect to the twist angle has yet to be fully mapped, and it remains unclear whether there exist optimal twist angles for observing these phenomena.…”

“…The flat bands found in the growing family of graphene-based moiré materials offer a valuable platform for studying the interplay of strong correlations and topology. , Twisted double bilayer graphene (tDBG), composed of two slightly relatively rotated sheets of Bernal bilayer graphene, has been the focus of significant research attention − owing to its exceptional tunability with perpendicular displacement field, − twist angle, and pressure . Compared with twisted bilayer graphene (tBLG), in which strongly correlated states occur over a very narrow range near the magic angle of ≈1.1°, tDBG hosts correlated states over a wider range of twist angles due to its lack of a singular “magic angle” from theory.…”